Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
607 PM EDT Sun Sep 27 2020
Valid 12Z Thu Oct 01 2020 - 12Z Mon Oct 05 2020
...Overview and Guidance/Predictability Assessment...
Model/ensemble guidance is agreeable and consistent in showing a
closed mid level low over the Aleutians, southeast Bering Sea, and
adjacent north Pacific Ocean. The low is surrounded by high
amplitude ridges over eastern Asia and western Canada. The slow
movement of the mean trough and closed upper low sets up a pattern
where the cyclones over the next week will track over similar
areas.
Guidance continues to improve in the clustering of solutions with
low moving north across the upper Aleutians/AK peninsula on Wed
into the southeast Bering Sea/coastal southwest AK Thu morning.
This means a forecasts consisting of a blend of the
ECMWF/GFS/UKMET should work to mitigate minor timing/intensity
differences.
There is developing better agreement that on Thu morning, a triple
point low develops over the north Pacific that moves north
towards the AK peninsula or adjacent Kodiak Island or upper
Aleutians, continuing a northwest motion into the southeast Bering
Sea on Sat and occluding. There is better than average agreement
among the 12z GFS, 12z ECMWF, 12z Canadian, and the 12z GEFS
Mean/ECMWF Mean on the cyclone development and track. The
multi-model/multi-ensemble blend was used to mitigate less than
normal differences, with less weighting of the ensembles than
usual.
At the end of next weekend into early next week, the models are
suggesting the possible development of a new cyclone in the north
Pacific that would drift north and possibly make it north into the
Gulf of AK on Sunday night 04 Oct into Monday 05 Oct.
The 00z Canadian/GFS/ECMWF and the GEFS and ECMWF Ensemble Means
are in better than average agreement on this system developing and
its location on Mon 05 Oct as it approaches the southern coast.
...Weather/Threats Highlights...
The pattern will favor increasing potential for heavy
precipitation from the AK Peninsula to the northern Panhandle.
The first surface low tracking northward towards Kodiak Island,
should advect moisture into the Gulf of Alaska and eventually the
southern Coast/northern Panhandle early next week, with gale force
winds to locally storm force likely to accompany the cyclone. This
system should produce moderate to heavy rain from the AK peninsula
and Kodiak Island, the Kenai peninsula, and coastal areas east as
far as the northern panhandle.
The next system will likely bring another round of moderate to
heavy rainfall Friday. The strength of the lows in the ECMWF,
GFS, Canadian, and German ICON favor another surge of high winds
and heavy rain for Kodiak Island to the southern coastal areas,
including areas from the Kenai peninsula east to the northern
panhandle. With the early week storm having made soils wetter or
even saturated, this storm
has potential to producing a flooding threat. The same would
apply to the low Oct 4-5, with another wind and flooding threat
possible.
Expect widespread coverage of above normal lows across much of the
Alaska mainland. The one area of below normal temperatures is
near the upper trough in the Aleutians.
Petersen
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html